The present invention relates to electrically addressable optically active matrix arrays, such as liquid crystal displays (LCDs) or spatial light modulators. More particularly, the invention relates to methods and apparatus for enhancing performance of such active matrix arrays.
Liquid crystal materials and other electro-optical materials often have asymmetric transition times, for example, the transition from a bright to a dark state can be different from the transition from the dark state to the bright state. In some examples, the transition time from state A to state B may be up to four times faster than the transition time from the state B to state A. In one mode of operation A may be bright and B may be dark, and in another mode B may be bright and A may be dark. It is generally the case that the transition time for one direction (A to B) can be accelerated by applying higher drive voltages, however the other transition direction (B to A) is limited by the physical and mechanical properties of the liquid crystal molecules. As such, the other transition direction cannot be accelerated by electronic means.
Faster frame update rates are highly desirable for display systems. For example, faster update rates decrease flicker (improving image quality) and relieve eyestrain. In field-sequential color systems, slow update rates lead to the objectionable “color breakup” effect, where successive red, green, and blue images are drawn too slowly for the human visual system to temporally fuse the images.
In a typical liquid crystal display (LCD), a series of pixels, each including liquid crystal material, are driven with drive voltages, in order to change the state of the material. More specifically, in a typical display addressing scheme, voltages are driven onto the pixel electrodes in a sequential scanning method to force transitions to a particular state, e.g. bright or dark. Often, several voltages are provided to the display at once to reduce addressing These pixel driving voltages may be continuous (analog), as used by companies such as Colorado Microdisplay, Inc., or binary (digital), as used by companies such as DisplayTech, Inc. There are also hybrid approaches where a digital pixel value is used as a selector to multiplex global analog voltages onto pixel electrodes.
A drawback to prior addressing methods is that they limited the performance of the LCD. One common factor in prior addressing methods is that the overall display update interval was determined by the sum of the matrix addressing time and the worst-case electro-optical material transition time. Generally, the longer the addressing and transition times, the slower the performance of the pixels and the LCD.
Attempting to increase the performance of an LCD despite the fixed addressing and transition times decreased image fidelity and lead to a phenomenon termed temporal crosstalk. Typically, the worst-case electro-optical material time must be used to determine performance of the LCD because the data displayed on the LCD may not valid until the very last pixel element that was addressed has transitioned to its final state, e.g. to A or to B. If the display were allowed to be viewed before the last state transition has been completed, the viewer would perceive an blend of the new pixel state or brightness and the previous frame's or field's pixel state or brightness.
One possible approach to reduce temporal crosstalk is blank the display while addressing the pixels. This approach necessitates a trade-off between brightness and contrast. If the display is blanked to a dark state, the average perceived brightness would decrease. If the display is blanked to a bright state, black pixels would appear bright for some of the frame time, increasing the perceived brightness of, and thereby decreasing contrast.
Temporal crosstalk also has undesirable effects in the field-sequential color mode of operation. Field sequential systems produce color images using a grayscale display and color illuminators (typically red, green, and blue). In this mode, a grayscale image corresponding to the red component of an image is drawn on the display and then the display is illuminated with a red light, from a light-emitting diode (LED) or with a bright lightbulb and a color filter. The process is repeated again for the blue and green image components. If the refresh frequency is sufficiently high, the eye will perceive uniform color.
The sequence of a particular field is therefore: (1) update the pixel voltages; (2) wait for the liquid crystal to transition; and (3) illuminate the device. If step (2) is too short and the LC material does not complete the transition, the current color component will be a blend of the previous color and the current color. For example, a bright green image has a dark red field followed by a bright green field followed by a dark blue field. Temporal crosstalk would result in a too-dark green followed by a too-bright blue. Therefore, color purity would be adversely affected.
In light of the above, what is needed are improved methods and apparatus for increasing performance of an LCD.